Radiosity OverView Part 1

Slide 1 : Title Slide.

This slide set will explain the radiosity method for computer
image generation and how the basic algorithm has been extended.

Slide 2 : Direct And Indirect Light.

Every surface in an environment is illuminated by a
combination of direct light and reflected light. The direct light
is light energy which comes directly from a light source or light
sources, attenuated only by some participating media (smoke, fog,
dust). The reflected light is light energy which, after being
emitted from a light source or light sources, is reflected off of
one or more surfaces of the environment.

When light energy is reflected from a surface it is attenuated
by the reflectivity of the surface, as some of the light energy
may be absorbed by the surface, and some may pass through the
surface. The reflectivity of a surface is often defined as its
color.

Slide 3 : Examples of Rendering Methods.

Three images are displayed on this slide, illustrating several
facets of computer image generation.

The image in the upper right corner was rendered with a
scanline rendering algorithm, where the ambient component of
light is approximated with a constant value. This results in even
shading, even in areas where shadows or less illumination would
be expected. No shadows are calculated.

The image in the middle and the image in the lower left corner
were both rendered with a ray tracing global illumination
algorithm. The image in the middle exhibits some of the
characteristics of a typical ray tracing algorithm: mirror-like
reflections and no ambient light component. Notice the hard-edged
shadows cast by the light source. The image in the lower left
corner retains the mirror-like reflection typical of a ray
tracing algorithm, and adds an accurate ambient component of
light, by allowing the rereflection of light energy through the
environment, as well as soft shadows.

Slide 4 : Diffuse Interreflection.

If a surface is defined to be a "diffuse reflector"
of light energy, any light energy which strikes the surface will
be reflected in all directions, dependent only on the angle
between the surface's normal and the incoming light vector. This
relationship is known as Lambert's law.

Light which is reflected from a surface is attenuated by the
reflectivity of the surface, which is closely associated with the
color of the surface. The reflected light energy often is
colored, to some small extent, by the color of the surface from
which it was reflected.

This reflection of light energy in an environment produces a
phenomenon known as "color bleeding," where a brightly
colored surface's color will "bleed" onto adjacent
surfaces. The image in this slide illustrates this phenomenon, as
both the red and blue walls "bleed" their color onto
the white walls, ceiling and floor.

Slide 5 : Introduction to Radiosity.

The "radiosity" method of computer image generation
has its basis in the field of thermal heat transfer. Heat
transfer theory describes radiation as the transfer of energy
from a surface when that surface has been thermally excited. This
encompasses both surfaces which are basic emitters of energy, as
with light sources, and surfaces which receive energy from other
surfaces and thus have energy to transfer.

This "thermal radiation" theory can be used to
describe the transfer of many kinds of energy between surfaces,
including light energy.

As in thermal heat transfer, the basic radiosity method for
computer image generation makes the assumption that surfaces are
diffuse emitters and reflectors of energy, emitting and
reflecting energy uniformly over their entire area. It also
assumes that an equilibrium solution can be reached; that all of
the energy in an environment is accounted for, through absorption
and reflection.

It should be noted the the basic radiosity method is viewpoint
independent: the solution will be the same regardless of the
viewpoint of the image.

Slide 6 : The Radiosity Equation.

The "radiosity equation" describes the amount of
energy which can be emitted from a surface, as the sum of the
energy inherent in the surface (a light source, for example) and
the energy which strikes the surface, being emitted from some
other surface.

The energy which leaves a surface (surface "j") and
strikes another surface (surface "i") is attenuated by
two factors:

the "form factor" between surfaces
"i" and "j", which accounts for the
physical relationship between the two surfaces

the reflectivity of surface "i", which will
absorb a certain percentage of light energy which strikes
the surface.